1. Field of the Invention
The present invention relates to a potentiometer.
2. Description of the Prior Art
In automobiles an EGR (exhaust gas recirculation) system is a known and effective technology to reduce emissions, particularly NO.sub.x (oxides of nitrogen), from the exhaust gas. The EGR system circulates an inert exhaust gas to the air intake side of the engine and, depending on a ratio between the amount of recirculated exhaust gas and the amount of intake air, or EGR ratio, the burning condition of fuel in the engine varies greatly, affecting the output of the engine, fuel cost and drivability. For optimum engine condition, a delicate and fine control is performed on the EGR system, such as increasing the EGR ratio under a driving condition where the amount of NO.sub.x is large and cutting the recirculation of exhaust gas when the exhaust gas recirculation is not needed. For this control, a technology has been realized in which the opening degree or lift of the EGR valve for recirculated exhaust gas is preselected for each driving condition and stored in a PROM as an output of a potentiometer, and in which the EGR valve is controlled by a microcomputer during engine operation so that the actually measured output of the potentiometer will agree with the output of the stored valve lift.
Since the EGR valve is operating at all times according to the driving pattern of the car, the EGR valve and the potentiometer, which measures the opening degree of the EGR valve for electrically determining the amount of recirculated exhaust gas, are required to withstand a significantly large frequency of operations. Other capabilities required of the EGR valve and the potentiometer are to withstand vibrations of a wide oscillation band from 20 Hz to 1 kHz coming from the engine and car body and accelerations as large as 30 G.
FIGS. 8 and 9 show conventional potentiometers as applied to the EGR system, disclosed in the Japanese Patent Publication No. Heisei 1-38245. In the figures, reference numeral 1' represents a potentiometer, 101 an EGR valve, 2 a potentiometer case, 3 a flange provided to one end of the case 2, 4 an O-ring installed in a groove 5 of the flange 3, 6 a fixing hole cut in the flange 3, and 7 a shaft slidably inserted in one end of the case 2. The shaft 7 is formed of such plastic as polyphenylene sulfide resin that has good lubricating performance, good rigidity and light weight.
Denoted 8 are connecting wires. Designated 102 is a housing of the EGR valve that has a passage 103 formed therein and a valve seat 104 installed in the passage 103. The housing is connected at an inlet 105 of the passage 103 with an engine exhaust pipe to receive exhaust gas and at an outlet 106 is connected with an air intake system of the engine. Designated 107 is a needle valve which has a valve 109 at the inner end to open and close the valve seat 104 to control the flow of the recirculated exhaust gas. At the upper end the housing 102 has a cup-shaped portion 102a formed integral therewith whose periphery is fitted with one end of an EGR valve case 110 with a diaphragm 109 clamped between. The end of the needle valve 107 is passed through and fixed to the center of the diaphragm 109. The diaphragm 109 and the case 110 together define an enclosed space, a negative pressure chamber 111, in which is installed a spring 113 between the case 110 and a press plate 112 mounted on the diaphragm 109. The case 110 is soldered at the upper end with a flange 114. One end of a case 2 of the potentiometer 1' is inserted airtightly into the end of the case 110. The flange 114 is formed with threaded holes at positions corresponding to the fixing holes 6 of the flange 3. Bolts are inserted and screwed into the threaded holes and the fixing holes 6 to fasten the two flanges 3, 114 together so that the potentiometer 1' is firmly mounted on the EGR valve 101 with an O-ring 4 clamped between. In this condition, the lower end of the shaft 7 is in contact with the upper end of the needle valve 107.
The case 2 has a through-hole 2a at one end for the shaft 7 and the other end of the case is open. A plate 9 is installed in the open end of the case 2. Between a recess 2b formed in the end of the case 2 and a recess 9a formed in the plate 9 is fixedly installed a printed circuit card 10 which has a resistor 10a printed on one side thereof. A terminal 11 which is folded to provide an elasticity and formed as a 3-pin terminal is inserted and fixed in the recess 9a. The end of the printed circuit card 10 is inserted between a guide projection 9b formed in the recess 9a and the terminal 11 against the elastic force of the terminal 11 to electrically connect the resistor 10a to the terminal 11. A voltage is applied between the ends of the printed circuit card. The terminal 11 is connected to the connecting wire 8.
Denoted 12 is a holder which is installed inside the case 2 so as to be slidable with respect to the inner wall of the case 2 in the direction of arrow C. The holder 12 engages with the end of the shaft and has the printed circuit card 10 passed therethrough. Around the insertion portion of the holder 12 through which the card 10 is passed are formed axially elongate guide portions 12a, 2b. Designated 12d is a gap between the printed circuit card 10 and the guide portions 12a, 12b. A metallic wiper 13 is attached to the holder 12 in such a way that it is in sliding contact with the resistor 10a of the printed circuit card 10 to collect current. A spring 14 is installed between the plate 9 and a spring seat 12c of the holder 12. The case 2, plate 9 and printed circuit card 10 as well as the shaft 7 and holder 12 are formed of resin.
Next, the operation of the potentiometer 1' and the EGR valve 101 of the above construction will be described. First, referring to FIG. 8, in the EGR valve the opening degree or lift of the valve 108 is determined by the difference between a force produced by the negative pressure in the chamber 111 that tends to push up the diaphragm 109 and a force of the spring 113 tending to push down the diaphragm 109. The valve lift in turn determines the amount of recirculated exhaust gas. The shaft 7 of the potentiometer 1' is pressed downwardly in FIG. 8 by the spring 4 through the holder 12. The end of the shaft 7 therefore is engaged with the end of the needle valve 107 so that the shaft 7 follows the motion of the needle valve 107. In other words, the shaft 7 exhibits a motion proportional to the opening degree of the valve 108. The holder 12 that engages with the shaft 7 moves with it, causing the wiper 13 to slide on the resistor 10a to divide the supply voltage thus producing an output proportional to the opening of the valve 108. Hence, when the valve 108 rests on the valve seat 109, closing the passage 103 and stopping the recirculation of the exhaust gas, the output of the potentiometer 1' is close to zero volt. When, on the other hand, the valve 108 is fully open to allow the maximum flow of recirculated gas, the output of the potentiometer 1' is close to the supply voltage. At an intermediate opening of the valve 108, the potentiometer output is proportional to the valve opening.
Because of the construction described above, the conventional potentiometer is subjected to vibrations of engine while it follows the motion of the EGR valve that responds to the driving pattern of the automobile. If the vibration is large, the holder 12 and the printed circuit card 10 move relative to each other in the gap 12d causing the wiper 13 to slide back and forth on the resistor 10a by a small margin, accelerating wear of the resistor 10a. When the negative pressure in the negative pressure chamber 171 increases and the difference between the opposing forces produced by the negative pressure and the spring 113 becomes small, the contact pressure between the valve 108 and the valve seat 104 decreases, causing a surging between the valve 108 and the valve seat 14 by vibrations. The impact of the oscillating motion of the valve 108 is transmitted through the needle valve 107 and shaft 7 to the holder 12, generating a rapid sliding motion of the wiper 13 and the resistor 10a. As a result, the resistor 10a rapidly wears. As the resistor 10a wears off, its resistance changes, which in turn causes substantial deviations in the correspondence between the resistance and the valve lift, making precise measurement of the amount of recirculated gas impossible. This means the exhaust gas recirculation is not performed correctly according to the running condition of the engine, increasing the amount of NO.sub.x in the exhaust gas.